Pasteurella multocida is a Gram-negative bacterial pathogen responsible for diseases of economic importance in a wide range of animal species, including atrophic rhinitis in pigs, bovine haemorrhagic septicaemia and fowl cholera, a severe disease of avian species (turkeys, chickens and ducks) which can present acute and chronic forms. The lack of cross-protection between different serotypes of P. multocida is a challenge for vaccine development. Commercial inactivated vaccines prepared from organisms grown in vitro fail to protect birds against heterologous field isolates. However, it is well established that live attenuated bacterial strains can be effective vaccines with better cross-protection. Live attenuated strains establish limited infection in the host and mimic the early stages of natural infection. Immune responses elicited by such vaccines are often of greater magnitude and longer duration than those induced by non-replicating immunogens. A large number of mutants obtained by the STM technique (Signature-Tagged Mutagenesis) have been evaluated at Merial for safety and efficacy in turkeys by ocular route of vaccination (2002, European Patent, application number: 02290861.0). Five of them showing the best safety/efficacy profiles were selected for full gene deletion in P. multocida of serotype A:3. Five single and two double deleted mutants without antibiotic resistance gene marker in their chromosome were obtained and tested at 10E+8 cfu/ml by ocular route for safety and efficacy studies in turkeys by homologous challenge. Despite some heterogeneity in safety and efficacy scores ranging from 55-100% and 20-90%, respectively, promising live attenuated candidates for clinical vaccine development were identified.